Field of the Invention
The present invention relates to a power supply for a magnetron, in particular but not exclusively for use with a magnetron powering a lamp.
Description of the Related Art
It is known that magnetrons can change mode unexpected, that is to say that unexpectedly they can stop oscillating at one frequency and start oscillating at another. Under these conditions, they can exhibit negative impedance. This can result in damagingly high current flow. For this reason, it is known that constant/controlled voltage power supplies are not suitable for magnetrons; constant/controlled current power supplies are usually used for powering them.
Anode voltages in magnetrons are high and measurement of both anode voltage and anode current are difficult.
In a previous power supply invented by the present inventor, measurement of both voltage applied to a converter in a magnetron power supply and current through the converter was utilised in a conjunction with a microcomputer to provide real time control of power supplied to a magnetron. The microcomputer was programmed to compute:    1. Power being consumed,    2. A difference from a desired power and    3. A difference between the power difference and the measured current.This second difference signal was used to control the converter. It should be noted that these three steps were executed in software. Unexpectedly, this power supply still suffered from a degree of instability, causing perceivable flickering of the light produced by its magnetron powered lamp.
Experience has now shown that the eye is extremely sensitive to light flicker in a magnetron powered plasma lamp. It has now been appreciated that the limited speed and resolution of the microprocessor output capability aggravated the perceived flickering. Additionally, two of the inputs to the microprocessor, namely the voltage applied to the converter and the current drawn through the converter are liable to be noisy and multiplication of the two noisy signals is believed to have contributed to the instability.
Simply filtering of the noise from the microprocessor reduces unacceptably the reaction time of the control circuit and contributes to instability, bearing in mind that fast reaction to changed magnetron conditions can be required. Accordingly a new approach was required.